Effects of adding monovalent alkali metal cations to Ca2+-depleted photosys
tem (PS)II membranes on the biochemical and spectroscopic properties of the
oxygen-evolving complex were studied. The Ca2+-dependent oxygen evolution
was competitively inhibited by K+, Rb+, and Cs+, the ionic radii of which a
re larger than the radius of Ca2+ but not inhibited significantly by Li+ an
d Na+, the ionic radii of which are smaller than that of Ca2+. Ca2+-deplete
d membranes without metal cation supplementation showed normal S-2 multilin
e electron paramagnetic resonance (EPR) signal and an S(2)Q(A)(-) thermolum
inescence (TL) band with a normal peak temperature after illumination under
conditions for single turnover of PSII. Membranes supplemented with Li+ or
Na+ showed properties similar to those of the Ca2+-depleted membranes, exc
ept for a small difference in the TL peak temperatures. The peak temperatur
e of the TL band of membranes supplemented with K+, Rb+, or Cs+ was elevate
d to similar to 38 degreesC which coincided with that of Y(D)(+)Q(A)(-) TL
band, and no S-2 EPR signals were detected. The K-induced high-temperature
TL band and the S(2)Q(A)(-) TL band were interconvertible by the addition o
f K+ or Ca2+ in the dark. Both the Ca2+-depleted and the K+-substituted mem
branes showed the narrow EPR signal corresponding to the S2YZ+ state at g=2
by illuminating the membranes under multiple turnover conditions. These re
sults indicate that the ionic radii of the cations occupying Ca2+-binding s
ite crucially affect the properties of the manganese cluster.